Add a Code Library snippet for CRP custom HTML rendering

[ajaydsouza]

Goal

Add a copy-and-paste example showing how to replace Contextual Related Posts default markup with the crp_custom_template filter while leaving CRP responsible for matching and selecting the related posts.

Existing coverage

crp-api-example.php renders a direct CRP_Query loop, and crp-add-categories.php changes one list item. Neither demonstrates full output replacement through crp_custom_template.

Deliverables

• Add contextual-related-posts/crp-custom-template.php or an equally clear filename under the display category.
• Register a uniquely prefixed callback on crp_custom_template and document $results as related post IDs and $args as the parsed display arguments.
• Render a small accessible wrapper and list with escaped permalinks, titles, and optional thumbnails, without assuming a particular theme.
• Return a non-empty HTML string when taking over the output and return null when the default renderer should continue.
• Add a matching Markdown companion under src/content/snippets/contextual-related-posts/ with installation guidance, requiresPro: false, and the hook reference.
• Explain that the filter applies to automatic output and explicit display methods such as the shortcode, widget, block, and manual calls.
• Include the caching caveat: returned custom HTML is cached when CRP HTML output caching is enabled, so request- or user-specific markup needs caching disabled for that call.

This is a reusable renderer example, not a YARPP-compatible yarpp-template-*.php filesystem mechanism. The CRP API documentation update is tracked separately in contextual-related-posts#229.